`Mp4Writer` receives all the tracks/samples and then it adds them to
corresponding `Track` object. As the `Track` object is primarily to hold
track specific data and `Mp4Writer` is responsible for interleaving related
logic, the call to `doInterleave` should be with `Mp4Writer`. The `Track`
object should just act like a data holding class.
This will also help in reusing `Track` class for fragmented MP4 implementation.
PiperOrigin-RevId: 590138002
0660b55ceb